1. Field of the Invention
The present invention is related to the field of electric wireline wellbore logging tools. More specifically, the present invention is related to a system for inserting electric wireline tools into a highly inclined wellbore using both pipe and coiled tubing.
2. Discussion of the Related Art
Electric wireline wellbore logging tools are used for, among other things, making measurements of various physical properties of earth formations penetrated by wellbores. Electric wireline tools are typically inserted into the wellbore by the force of gravity, and are returned to the earth's surface by means of an armored electrical cable attached to the tool. The cable is drawn by a winch or similar spooling apparatus.
Certain wellbores are drilled so as to have very large inclination from vertical over at least a portion of the wellbore. Other wellbores can have a section which is substantially horizontal. Gravity cannot be used to insert logging tools into a highly inclined or horizontal portion of a wellbore, so various methods have been devised to insert logging tools into such highly inclined wellbores. For example, "Wireline Conveyance Systems", Atlas Wireline Services, Houston, Tex., 1991, describes several methods of inserting logging tools into highly inclined or horizontal wellbores.
One of the methods known in the art for inserting logging tools into highly inclined or horizontal wellbores is to attach the tools to the end of a pipe comprising threaded sections, and to lower the pipe into the wellbore by attaching additional sections to the pipe. After the tools are inserted to a desired depth in the wellbore, the electrical cable is attached to the tools by pumping the cable through the center of the pipe until the cable latches on to a special connector disposed at the top of the tools. The cable is inserted into the center of the pipe from the outside of the pipe through a device called a "side entry sub". A side entry sub is a short section of pipe having a sealable opening through a side wall of the section of pipe which enables passage of the cable through the wall of the sub. The side entry sub is typically assembled to the pipe at a substantial distance below the upper end of the pipe. Assembled in this position, the side entry sub enables raising the logging tools within the wellbore by removing sections from the pipe simultaneously with spooling of the cable as the tools are raised in the wellbore. In this way, portions of the wellbore can be measured with the logging tools without repeated insertion and removal of the cable from the inside of the pipe.
A drawback to using pipe to convey the logging tools is the presence of the cable outside the pipe from the position of the side entry sub up to the earth's surface. In some cases control of fluids which may be present in the wellbore requires using equipment located at the earth's surface designed to seal an annular space between the pipe and the wellbore. In other cases it is necessary to maintain fluid pressure on the wellbore from the earth's surface in order to obtain valid measurements from the tools in the wellbore. Cable disposed outside the pipe disturbs the operation of the sealing equipment and makes it difficult to seal the wellbore for maintaining fluid pressure.
Tools can also be inserted into the wellbore by using a coiled tubing having a coaxially inserted electrical cable. Because the cable is coaxially inserted through the coiled tubing, it is possible to seal the annular space between the wellbore and the coiled tubing with equipment similar to that used to seal the annular space outside the sectioned pipe.
One of the difficulties with coiled tubing used in highly inclined or horizontal wellbores, is that frictional force which develops between the wellbore wall and the coiled tubing as a result of the tubing contacting the lower wall of the wellbore can sometimes exceed the buckling strength of the coiled tubing. When the buckling strength of the tubing is exceeded, the tubing can kink or bend so that it becomes impossible to push the tools further into the wellbore.
It is an object of the present invention to provide a system for inserting logging tools into a highly deviated or horizontal wellbore using threaded pipe in which the annular space between the pipe and the wellbore can be sealed.
It is a further object of the present invention to provide a system for inserting logging tools in a highly deviated or horizontal wellbore which is resistant to failure caused by frictionally induced buckling of coiled tubing.